```python
from typing import Dict

def trace_relay(links: Dict[str, str], start: str) -> list:
    """Walks through handoffs from 'start' to the empty string, checking for invalid links and circular paths."""
    
    if not isinstance(links, dict):
        raise ValueError("links must be a dictionary.")
    if start not in links:
        raise ValueError(f"Start post '{start}' is not in the links dictionary.")
    if links.get(start) == "":  # Start at the end post
        return [""]
    
    visited = set()
    handoffs = []
    
    current_post = start
    
    while current_post != "":
        if current_post in visited:
            raise ValueError("The watch comes back to a post already visited.")
        
        visited.add(current_post)
        handoffs.append(current_post)
        
        next_post = links.get(current_post)
        if next_post is None or not isinstance(next_post, str):
            raise ValueError(f"Handoff from '{current_post}' is invalid.")
        
        current_post = next_post
    
    return handoffs
```